Typically, an LCD is mainly exemplified by a thin film transistor (TFT) LCD, which has excellent color reproducibility and is thin. A general TFT-LCD is known to be a transmissive LCD, which includes a TFT array substrate, serving as a lower substrate, and a color filter substrate, serving as an upper substrate.
In the TFT-LCD, a backlight unit positioned under the lower substrate is used as a light source. Only about 7% of the light produced from the backlight unit is actually radiated onto a screen while it passes through the TFT array substrate and the color filter substrate. Thus, with the goal of fabricating an LCD having high luminance, since the backlight unit is required to be brighter, the power consumption of the backlight unit is increased. Further, a battery suitable for use in supplying power to the backlight unit has drawbacks, such as heaviness and a limited service time.
To solve these problems, research and development on reflective LCDs that do not use backlight units is being conducted these days.
Since the reflective LCD is operated using external light, it may drastically decrease the power consumption of the backlight unit, and thus it may be portably used for a long period of time.
The reflective LCD is composed of a reflection plate or reflection electrode having opacity and reflection properties. The reflective LCD realizes an image in such a manner that external light is passed through a color filter substrate, reflected through a reflection plate or reflection electrode provided to the lower substrate, and then transmitted through the color filter substrate. In this way, the reflective LCD suffers from drastically lowered luminance because external light is passed two times through the color filter, and thus light transmittance is decreased.
Therefore, the intention is to overcome the above-mentioned problems by making the color filter substrate thin so as to achieve high transmittance and low color purity. However, there are limitations on the fabrication of the color filter to a predetermined thickness or thinner due to the properties of the resin used in the color filter.
Accordingly, research and development on LCDs that have no color filter substrate and use cholesteric liquid crystals, which are able to selectively reflect or transmit light, has been recently conducted. Such cholesteric LCDs, exemplified by a reflective LCD using Bragg reflection, are now drawing attention as a low power consumption LCD thanks to the bistable orientation state (bistability) thereof.
The liquid crystal molecules generally have a liquid crystal phase that varies with the texture and composition thereof, the liquid crystal phase being affected by the temperature and concentration. The liquid crystals or liquid crystal phases, which have been thoroughly studied and applied to date, include, for example, nematic liquid crystals, in which liquid crystal molecules are orderly aligned in a predetermined direction. Such nematic liquid crystals are generally applied to commercially available LCDs.
In contrast, cholesteric liquid crystals are liquid crystals having a distorted liquid crystal molecular axis or having a distorted director configuration of the nematic liquid crystals because the nematic liquid crystals are mixed with chiral molecules, in which the original molecular phase and the reflected molecular phase are different from each other.
Further, the nematic liquid crystal phase is composed of liquid crystal molecules orderly aligned in the predetermined direction.
In contrast, the cholesteric liquid crystals have a layered structure, in which the liquid crystals of respective layers manifest typical nematic ordering. However, interlayer liquid crystals are arranged to be rotated in one direction, and interlayer reflectance varies as a result of such rotation. The difference in reflectance may result in the exhibition of color through reflection and interference of light.
FIG. 1 is a view showing the principle of the cholesteric LCD.
The cholesteric liquid crystals are twisted into a helical structure. The length required for the director to rotate through 360° is referred to as the pitch, which is a parameter determining the hue of the cholesteric liquid crystals. The cholesteric LCD has a continuous arrangement of layers of liquid crystals having the same pitch, and may selectively reflect light of a wavelength equal to that of the helical pitch length according to Bragg's law.
As such, the selectively reflected central wavelength (λ) is represented as a function (λ=n(avg)×pitch) of the pitch and the average refractive index (n(avg)) of the cholesteric liquid crystals. For example, in the case where the average refractive index is 1.5 and the pitch of the cholesteric liquid crystals is about 370 nm, the reflective central wavelength is determined to be about 555 nm. When external white light enters the cholesteric liquid crystals, as shown in FIG. 1, green light is reflected, while red light and blue light are transmitted and absorbed by the absorbing layer, leading to exhibition of green color. Moreover, a wavelength width ((( ) is obtained by multiplying an anisotropic refractive index ((n) and pitch. The selective reflection properties of the cholesteric liquid crystals are shown in the graph of FIG. 2.
The selectively reflected central wavelength and the wavelength width thereof depend on the pitch and anisotropic refractive index of the cholesteric liquid crystals. In general, red, green and blue colors are determined by the pitch of the cholesteric liquid crystals.
In order to realize full color, the cholesteric LCD is constructed by superimposing three cholesteric LCDs having different pitches as shown in FIG. 3. The respective cholesteric liquid crystal layers are controlled with respect to the pitch so as to exhibit specific colors thereof. Additionally, ITO (indium tin oxide) and a glass substrate are provided between the cholesteric liquid crystal layers, and an absorbing layer is provided beneath a lower substrate. As such, power is separately applied to respective cholesteric liquid crystal layers. When power is applied, the liquid crystal orientation of the cholesteric liquid crystal layer has a focal conic texture, and thus the color corresponding to that layer is transmitted and absorbed. On the other hand, when no power is applied, the liquid crystal orientation of the cholesteric liquid crystal layer has a planar texture and therefore the color corresponding to that layer is reflected.
FIG. 4 is a view showing the exhibition of full color by the cholesteric LCD of FIG. 3. When power for a red layer and a blue layer is turned on and power for a green layer is turned off, green light among incident white light is reflected, and red light and blue light thereof are transmitted and absorbed by the absorbing layer. In this way, when power to be applied to respective layers is appropriately controlled, the reflectance of individual colors (red, blue, green) may be adjusted, therefore realizing full color.
However, as in FIG. 3, when the three LCDs are superimposed, they are thickened and have inefficient driving properties, thereby increasing the cost of fabricating color LCDs.